Apparatus for coating conductive articles

ABSTRACT

Apparatus for electrical deposition of a coating from an aqueous solution simultaneously on a plurality of conductive articles including a conductive container within a tank holding the aqueous solution for receiving a plurality of articles to be coated, and means for shifting the position of the articles relative to each other at predetermined time intervals.

United States Patent [191 Field of Search ..204/286, 287, 297, 204/300, 299, 181

Haney Apr. 17, 1973 APPARATUS FOR COATING [56] References Cited 2 CONDUCTIVE ARTICLES UNITED STATES PATENTS [75] Inventor: Eugene E. Haney, Middletown, Ohio I 1,798,994 3/1931 Whalm ..204/287 [73] Assignee: Armco SteeLMiddle'town, Ohio 2,766,201 10/1956 2,841,547 7/1958 [22] Flled: Mar. 1, 1971 3 399 12 19 3 21] Appl. No; 119,718 3539489 2 Related US, Application Data Primary Examine-John R Mack 1 v 0 Assistant Examiner-A. C.Prescott [62] Division of Sero- 78 1366- 8, Pat. No. ArzorneyMelville, Strasser, Foster & Hoffman [57] ABSTRACT [52] Cl. ..204/300, 204/181, 204/286, Apparatus for electrical deposition of a coating from v 204/287 204/297 an aqueous solution simultaneously on a plurality of [51 lllli. .C|.;....... ..B0lk 5/02, C231) 13/00 conductive articles including a conductive Container 1 within a tank holding the aqueous." solution forreceiving a plurality of articles ,to be coated, and means for shifting the position of the articles relative to each other at predetermined time intervals 3 Claims, 1 Drawing Figure mu. I

PATENIEDAPRIYIQYB 3.728.247

INVEENTOR/S EUGENE E. HA NEY W ATTORNEYS APPARATUS FOR COATING CONDUCTIVE ARTICLES CROSS REFERENCE TO RELATED APPLICATIONS This is a division of application U. S. Ser. No. 781,049, filed on Dec. 4, 1968 in the name of Eugene E. Haney and entitled Method for Coating Conductive Articles", now U. S. Pat. No. 3,316,392

BACKGROUND OF THE INVENTION This application relates generally to a process for electrically depositing a coating on electrically conduc tive articles. Such systems have been variously called electrocoating, electrophoresis or electropainting. The electrically conductive articles or surfaces found applicable to the practice of the invention include the metals iron, steel, aluminum, zinc, brass, bronze, titani um, cadmium and columbium, although the method and apparatus may be used to coat any material having the ability to attract coatings ionized in an aqueous solution.

According to present techniques, the article to be coated is immersed in a specially formulated solution. A paint solution, for example, generally includes a resin, a suitable pigment, and a solubilizing agent.

While either anodic or cathodic electrocoating is possible, the anodic techniques are most highly developed. That is, the article to be coated is given a positive charge while the paint container or-other electrode in the paint bath is given a negative charge. The resin and pigment components of the paint carry negative charges and will migrate toward the article itself, while the solubilizer carries a positive charge.

It is now known that these electrically deposited coatings have certain very unusual characteristics and advantages. In the first place, as indicated above, the particles of pigment and resin migrate toward the surface of the article being coated, where they are irreversibly deposited as a coating. As this coating builds up on the surface of the article being coated, that portion of the surface becomes less conductive. This of course tends to limit the charge on the article only to uncoated surfaces. Thus, the application of a paint coating by this method provides an absolutely uniform thickness at all points on the article. This characteristic of the operation is often referred to as a self-limiting factor. That is, for a given set of conditions,'a predetermined quantity of paint will be deposited on all portions of the article, and when that level has been reached, substantially no further accumulation will occur.

,Thus, the uniform and accurately controllable coating thickness is a great advantage to this process.

The instant invention will be described in its relation- .ship to the electrocoating of fasteners, although it will much to be desired. Prior coating techniques, including experimental studies of the electrophoretic process, have met with great difficulty in prior attempts to produce pre-coated articles which will be later sub jected to mechanical forces and chemical attack.

It is believed that this difficulty is due primarily to two factors. First of all, considering for example the threaded fastener art, a threaded product by nature is extremely difficult to paint evenly due to surface tension of conventional coatings containing solvents. That is, screw threads, the head of a bolt or nut, or the slotted head of a typical screw present sharp angular surfaces, and conventional painting processes such as spraying or dipping do not result in uniform coverage.

Secondly, and perhaps more importantly, such fastener products as are used in industry are typically driven or installed by means of power equipment, such as pneumatic screw drivers, nut runners, and the like. It is well known in the industry that such fastener driving equipment will chip or scratch the: paint from fasteners painted in any conventional manner.

In addition, such commercial fasteners are generally very small and utilized in large numbers, so that the painting operation must be set up to handle the job with a minimum of labor and individual part handling. That is, electropainting is generally considered impractical because, according to conventional practice, each article being coated is separately charged; this becomes economically impractical in terms of the quantity of products to be coated according to this application.

Keeping the foregoing discussion in mind, it is an object of this invention to provide an apparatus which will, without individual handling, provide an accurate and controllable coating on a plurality of individual metallic articles.

SUMMARY OF THE INVENTION This invention contemplates a container within a' suitably formulated bath'into which thepreviously prepared articles to be coated are passed The con tainer maybe comprisedby two or more conveyors. The articles to be coated are first loaded into one of the conveyors which will transport them through a portion of the bath and discharge them into a second conveyor. The second conveyor or subsequent conveyors carry these articles through the remainder of the bath and discharge them for further processing. l

DESCRIPTION OF THE DRAWING The drawing is a schematic view of the bath and one form of the conveying arrangement contemplated by this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT water rinse has proved highly satisfactory. Under certain conditions, such as when a heavy grease, drawing compound or oxide is on the product, it may be necessary to use a solvent degreasing operation and/or an acid pickle.

Following the cleaning steps, a conversion coating is applied to the surface of the article to be coated. Two types of conversion coatings have been found satisfactory. On the one hand, particularly for articles which have been metallic coated as by galvanizing or the like, the article ispassed through a phosphate bath (such as zinc phosphate), followed by a chromate seal.

A second conversion coating which in many instances is to be preferred calls for passing the article to be coated into a bath consisting of an organic fluoride containing chromic acid and zinc dust. While this material is more expensive in base form, it does not require a chromic acid rinse, and hence one of the stages of a conventional phosphating line can be eliminated. It may also be applied at room temperature.

Following the application of the conversion coating, the article is subjected to heat to make it completely dry. prior to painting. The effect of this prebaking operation is not precisely known. Some have theorized that the paint will adhere better if the product enters the bath dry. While not intending to be bound by theory, applicant believes that this prebaking operation drives out occluded gases from a prior coating or even from the metal itself, and drives out the water of crystallization from the phosphate or chromate if these are previously applied. In any event, it has now been discovered that this prebaking step gives superior results in terms of corrosion resistance and strength of the bond between the paint film and the article itself.

This prebaking step should be carried out at a temperature and for a time generally equal to the time and temperature of the final paint baking step described presently. Preferably, the maximum temperature attained by the articles in the prebaking step will be at least equal to the maximum temperature attained by the articles during the final baking. At the present time, and with presently known paint systems, prebaking times and temperatures will be on the order of at least 4 minutes at temperatures from 300 to 500 F.

The electropainting step is carried out in a bath containing a suitably formulated water base organic paint. The precise formulation of the paint utilized is beyond the scope of this invention. For present purposes, it is sufficient to say that many paint manufacturers have formulated paints for an electropainting operation. These include several different resin systems such as oleoresinous materials, acrylics, epoxies, and others.

Referring now to the FIGURE, the paint bath will be maintained in the tank indicated generally at 10. The normal level of paint in the tank is at 12.

Suitably mounted within the tank is the first totally immersed conveyor indicated generally at 14. This conveyor includes a plurality of containers 16, each of which is secured to the flexible drive means 18. The

drive means 18 of course forms an endless conveyor about the drive pulleys 20 and 22. It will be understood that this conveyor is driven through any suitable drive train at the desired speed selected in the light of the paint system utilized.

Also within the tank 10 is the secondor exit conveyor 24. As will be readily seen, the construction of this conveyor is substantially identical to the construction of the first conveyor, except that this conveyor is inclined upwardly so that a portion of it is disposed above the normal paint level 12 of the bath. (Corresponding parts of the conveyor 24 have been indicated in the FIGURE by primed reference numerals.)

Adjacent one end of the first conveyor 14 is the loading hopper 26. This hopper is provided with doors or the like (not shown) for releasing, at predetermined intervals, the desiredv quantity of articles to be coated into the containers 16.

Disposed between the first and second conveyors is the interchange hopper indicated at 28. As the containers on the first conveyor reach the right hand limit of travel, they in effect dump their contents into the interchange hopper which effectively funnels the material into the containers 16 of the exit conveyor.

It will be apparent that the operation of the first and second conveyors described above and the interchange hopper is effective to move the articles in the containers of the conveyor with respect to each other a predetermined time after immersion in the paint bath. It has been discovered that when a large quantity of items is randomly deposited in a container of this kind, it is possible to trap air bubbles or to have contact. points between adjacent articles which prevent full and effective coating of the entire article. By the provision described above, this invention insures uniformity of the coating even though a relatively large number of small articles is being treated simultaneously in a container.

Maintained within the bath are the two cathode grids 30 and 32. The anode or positive connection from the source of electric current (not shown) is effected by a brush type contact to the first and second conveyors l4 and 24 described earlier. After emergence from the paint bath, the articles are preferably subjected to a water rinse and then baked in accordance with the requirements of the paint being used. The term baking as used herein shall be understood to include other methods of completing the polymerization of the organic coating such as air curing at ambient temperature or radiation curing.

To facilitate a full understanding of this invention, the following specific example of a commercial run is given. Two hundred pieces of 541-14 hex head selftapping electrogalvanized steel screws were cleaned in a mild alkaline cleaner commercially available under the designation PARKER 346 as sold by the Parker Division, Hooker Chemical Corporation. This cleaner was diluted in the proportion of 6 ounces per gallon of water, and the bath was maintained at a temperature of 170 F. The articles were maintained in this bath at this temperature for a period of5 minutes.

Following a water rinse after the alkaline cleaner, the articles were immersed in a zinc phosphate bath available from Parker Division, Hooker Chemical Corporation under the trade designation BONDERITE 378. Total acid of the solution equaled 15 points. The bath maintained at a temperature of 160 F and the articles were immersed in this bath for a period of 5 minutes.

Following the phosphate bath, the articles were subjected to a chromic acid rinse (one-tenth percent by weight) at F. for 30 seconds.

This in turn was followed by a double water rinse. The first rinse was with ordinary .tap water, while the second was with deionized water in order to prevent contamination of the paint during the subsequent steps. The product was then subjected to a preliminary baking at 400 F. for a period of5 minutes.

The particular paint used in this example was available under the designation JD 1 69, available from Jones- Dabney Division of Celanese Coatings Company. As supplied by the manufacturer, this paint contained 40 percent non-volatiles. Before use, the paint was diluted with deionized water at the rate of one part paint to three parts water.-

When finally ready for use, the paint contained percent non-volatiles and was slightly alkaline (pH 8.0).

The paint bath was maintained at a temperature of 75 F. and the electrical charge was maintained at 120 volts. The dwell time for the articles in the bath was 1 minute.

The final baking of the coated articles was at a temperature of 400 F; for 30 minutes.

The resulting products from the foregoing run yielded a paint coating 0.7 mils thick. These products were tested for corrosion resistance and for ability to hold paint under mechanical driving. The articles passed these tests with no failures.

It is believed that the foregoing constitutes a full and complete disclosure of this invention. Since many modifications can be made within the scope and spirit of this invention, no limitations are expressed or implied except as specifically set forth in the claims which follow.

The embodiments ofthe invention in which an exclusive property or privilege is claimed are defined as follows:

1. In the electrodeposition of a coating from a solution, simultaneously on a plurality of articles, the improved continuous apparatus comprising:

a. a tank for containing a supply of said solution;

b. first means within said tank for receiving said plurality of articles and retaining themover a period of time;

. second means for subsequently receiving said plurality of articles and retaining them for a period of time;

. means for passing said plurality of articles from said first means to said second means so as to cause a repositioning of said articles with respect to each other, permitting the escape of occluded air bubbles; and

. means for passing an electric current through said first and second receiving means and said solution, whereby to deposit. said coating uniformly throughout the surface of each of said articles.

2. In a tank for the elect'rodeposition of a coating simultaneously on a plurality of conductive articles, the improved continuous apparatus comprising:

a. first means for receiving said plurality of articles and retaining them for a period of'tirne; second means for subsequently receiving said plurality of articles and retaining them for a period of time; and means for passing said plurality of articles from said first means to said second means so as to cause a repositioning of said articles with respect to each 0 her, and to permit the escape 0 oc- 

2. In a tank for the electrodeposition of a coating simultaneously on a plurality of conductive articles, the improved continuous apparatus comprising: a. first means for receiving said plurality of articles and retaining them for a period of time; b. second means for subsequently receiving said plurality of articles and retaining them for a period of time; and c. means for passing said plurality of articles from said first means to said second means so as to cause a repositioning of said articles with respect to each other, and to permit the escape of occluded air bubbles.
 3. The apparatus claimed in claim 1 wherein said first and second means each comprises a conveyor, said conveyors each including a plurality of individual containers, hopper means for guiding said articles into the containers in one of said conveyors, and hopper means for receiving articles discharged from said one of said conveyors and guiding them to successive containers of said second conveyor. 